The present invention relates to exposure control and more particularly to a method for controlling the exposure of a photosensitive resist illuminated by a pulsed laser.
In the manufacture of semiconductor integrated circuits, the desire to define ever smaller features using microlithographic techniques is leading to the use of shorter and shorter wavelengths. The features are typically formed by exposing a photosensitive resist coated on a semiconductor wafer using a step and repeat projector. Increasing interest is presently being shown in the use of krypton fluoride (KrF) excimer lasers as a light source. These lasers are typically operated in a pulsed mode and are becoming available at greater power levels, both on a per pulse and total average energy basis.
In order to minimize the time spent exposing the resist at each position in the step and repeat process, the availability of increased power is generally desirable. Heretofore, exposure control has been effected by measuring the relative illuminating energy provided by each successive laser pulse and then terminating the exposure when the accumulated value reaches the desired exposure level. However, when the energy delivered by each laser pulse becomes a significant percentage of the total desired exposure, it is not possible to control the exposure with acceptable accuracy merely by terminating the illumination after that pulse which causes the accumulated exposure to reach the desired exposure level.
Among the several objects of the present invention may be noted the provision of a novel method of exposure control; the provision of such a method which is adapted to control the exposure of photosensitive resists illuminated by a pulsed laser; the provision of such a method Which provides precise exposure control; the provision of such a method which will provide precise exposure control when the energy provided by each laser pulse is greater than the acceptable deviation allowed in the overall exposure; the provision of such a method which facilitates high throughput in a step and repeat exposure system; the provision of such a method which is highly reliable; the provision of such a method which may be easily implemented by apparatus which is relatively simple and inexpensive. Other objects and features will be in part apparent and in part pointed out hereinafter.